1. Field of the Invention
The present invention relates generally to probe cards, and more particularly, to an improved probe card which permits correct needle pressure to be provided by probe needles of different heights. The invention further relates to a method of testing wafer using such a probe card.
2. Description of the Background Art
Generally in the process of manufacturing ICs and LSIs, after manufacturing a number of wafer chips on a single substrate, there is provided a wafer testing step to test whether individual chips are non-defective or defective prior to slicing them on a chip-basis. In such wafer testing, a probe card is connected to a device called prober and the probe needle of the probe card is set in contact with a prescribed electrode (pad) of a semiconductor chip. After making the probe needle contact with the semiconductor chip, fixed pressure (referred to as "needle pressure") is applied between the probe needle and the pad (the operation is referred to as "overdrive".) By the overdrive, the probe needle slides on the pad surface to remove aluminum oxide on the pad surface. Thus, aluminum which was under the aluminum oxide is electrically connected with the probe needle.
FIG. 14 is a cross sectional view showing a probe card using a conventional cantilever type probe needle disclosed by Japanese Utility Model Laying-Open No. 57-146340. The probe card includes a single printed circuit board 1 (hereinafter referred to as "substrate"). There is an opening 2 in the center of substrate 1. A plurality of probe needles 3 are radially provided on the bottom surface of substrate 1 having their tip ends in coincidence. The roots of probe needles 3 and a contact portion for connector connection (not shown) at an end of the printed circuit board are connected with one another by printed interconnections or wires.
FIG. 15 shows the relation between the deflection and force (F) of the cantilever type probe needle. Herein, the deflection of the probe needle refers to the height of the tip end of probe needle 3 from the surface of substrate 1 as shown in FIG. 16. Herein, force (F) refers to needle pressure (F) applied on the probe needle in the height-wise direction overdrive.
Referring to FIG. 15, a linear relation is established between the deflection and the force of the probe needle. Generally, in an overdrive of about 100 .mu.m, a needle pressure of about several g (for example, 7 g) is applied on the probe needle. Thus, the pad and the probe needle are electrically connected for testing the wafer.
The positional relation between the probe needle and the pad is critical in each of the directions of length, width and height, each of which requires precision as high as about .+-.10 .mu.m. In the future high density ICs, higher precision will be requested. Today, the positional relation between a probe needle and a pad is adjusted manually. By such manual operation, the height of the probe needle (the distance between the tip end of the probe needle and the bottom surface of the substrate) cannot be readily adjusted.
FIG. 17 is a view showing the concept of a presently used cantilever type probe needle. The entire length of a small probe needle is about in the range from 5 mm to 10 mm. The length of the bent needle tip portion is about 200 .mu.m. The diameter of the root of the probe needle is about 200 .mu.m.phi.. The diameter of the tip end of the probe needle is about 35 .mu.m.phi.. The probe needle is continuously tapered from its root toward its tip end. The diameter of the probe needle at the bent point is about 80 .mu.m.phi..
Since a number of probe needles are usually provided to a substrate, and the tolerance of variation in the height of probe needles is desirably about .+-.15 .mu.m, but the complexity of the manual adjustment keeps the tolerance at the level of about .+-.30 .mu.m.
If the variation in the height of the probe needles is large, correct needle pressure cannot be obtained. In addition, the large variation could lead to damages to ICs or makes it difficult to correctly determine the defectiveness and non-defectiveness of the ICs. Furthermore, as the pad is reduced in size in the future, when a probe needle is contacted with a pad and slided over the pad surface in an overdrive of about 100 .mu.m, the needle tip could travel off the surface of the pad, thus tarnishing the reliability of the IC.
Furthermore, there is a demand for manufacturing probe cards less costly by simplifying the operation of adjusting probe needles. Under the circumstances, how to solve the variation in the height of the probe needles is a major problem to be solved.